This invention relates to light-weight piston assemblies and, more specifically, to improved piston assemblies having piston bodies formed of light-weight metal and hard metal ring groove protection bands.
Prior art of possible relevance includes French Pat. No. 634,700 issued Nov. 30, 1927 to Ceskomoravska-Kolben Akciova Spolecnost and the following U.S. Pat. Nos.: 1,547,687 issued July 28, 1925 to Rohwer; 1,547,737 issued July 28, 1925 to Daiber; 2,266,192 issued Dec. 16, 1941 to Grieshaber; 2,349,919 issued May 30, 1944 to Starr; 2,361,095 issued Oct. 24, 1944 to Harrah; 2,398,577 issued April 16, 1946 to Bratzel; 2,478,179 issued Aug. 9, 1949 to Brockmeyer; 2,759,461 issued Aug. 21, 1956 to Maybach et al; 2,807,247 issued Sept. 24, 1957 to Cramer; 3,215,130 issued Nov. 2, 1965 to Maier; 3,380,556 issued Apr. 30, 1968 to Whitehead; and 3,385,175 issued May 28, 1968 to Meier et al.
The advantages of light-weight piston assemblies in reciprocating mechanisms, particularly engines, have long been recognized. In general, such piston assemblies are formed principally of light-weight metal such as aluminum or aluminum alloys and are typically provided with a ring groove protection band formed of a hard metal, such as cast iron, which carries one or more compression rings. The use of such bands has been necessary to provide long life to the piston assemblies in that, as is well known, during operation of reciprocating mechanisms embodying such piston assemblies, the minute movement which the piston rings undergo during the operation of the reciprocating mechanism would cause rapid deterioration of ring-receiving grooves if the grooves were formed directly in the light-weight, and generally softer, piston body.
One approach to the manufacture of such composite piston assemblies is to cast the ring groove protection band in the piston body. This approach requires that the material of which the body is formed have a coefficient of thermal expansion very nearly equal to that of the material forming the band so that the band will not loosen within the body as operating temperatures of the reciprocating mechanism change during operation.
As a practical matter, this approach also requires that very sophisticated inspection techniques be employed to check the soundness of the metallurgical bond between the piston body and the band immediately after fabrication and before installation in a reciprocating mechanism to ensure that the bond will not fail and shorten the useful life of the mechanism.
A variety of other approaches have been used, as exemplified by the above cited prior art. For example, frequently, ring grooves are carried by portions of the piston crown which is formed of a hard metal and secured by any of a variety of means to a light-weight skirt. This approach tends to minimize the advantages accompanying light-weight piston assemblies in that, in lieu of a relatively small hard metal band, the entire crown is formed of a hard and relatively dense metal so that a heavier piston assembly results.
Moreover, when such an approach has been employed, generally relatively short threaded fasteners have been used to secure the crown to the piston skirt and such threaded fasteners are easily overstressed when tightened to the degree necessary to ensure that "creep" between the parts will not occur during operation. Such overstressing inevitably results in premature failure of the assembly.
The above-identified Starr patent employs still another approach employing a single elastic washer which is clamped against a band, the washer being intended to store a clamping energy to be directed against the band to hold the same in place. This approach has not met with a great deal of success since the washer is easily overstressed if it is sufficiently stiff and if not stiff, cannot store sufficient energy to maintain the parts in a solidly clamped configuration.